NAND flash memory is known as one type of nonvolatile semiconductor memory. The cell structure of a NAND flash memory is a stacked gate structure with a first insulating film, a charge storage film, a second insulating film, and a control gate electrode stacked on a semiconductor layer in this order. The first insulating film is also called a “tunnel insulating film.” The second insulating film is called an “intergate insulating film” when the charge storage film is a floating gate electrode formed of polycrystalline silicon, and called a “block insulating film” when the charge storage film is an insulating film that stores charge.
Conventionally, the first insulating film and the second insulating film have been formed of silicon oxide or silicon oxynitride. However, as flash memories tend to decrease in size and increase in capacity, the interference between cells becomes more remarkable in rocket-type cells. For this reason, the employment of a planar cell structure instead of the rocket-type cell structure is being studied. However, silicon oxide or silicon oxynitride that has been conventionally used to form the second insulating film is, in the case of a planar cell structure, insufficient to ensure the coupling ratio and to maintain the insulation between the floating gate electrode or charge storage film and the control gate electrode. Therefore, a material of even higher dielectric constant needs to be used. Due to its high dielectric constant, wide bandgap, and high thermal stability, the application of a lanthanum aluminum silicon oxide film to the second insulating film is pursued.
However, the requirements of the second insulating film are not only a low leakage current but also a high breakdown voltage and a low charge-trapping capability. It is therefore important to improve the reliability of the high-k material used to form the second insulating film. Besides ensuring the reliability of the second insulating film, preventing the degradation of the first insulating film caused by write and erase operations is necessary in order to improve the reliability of the memory cell itself. As has been described, a nonvolatile semiconductor memory having highly reliable memory cells is required.